The full-page ads show a tide of young men and a flood of young women smiling and marching across Memorial Bridge with their personal effects. The ad says "Washington's most successful single men/women will be moving out of town."

The ad campaign is for a cooperative complex in Rosslyn called River Place--formerly known as Arlington Towers--and they say that efficiencies there start at $29,550 and one-bedroom units at $46,650. Promised are "the best views of Washington . . . lush landscaping and flowing streams" and a pool.

What the ads do not say is that the newly converted River Place cooperative sits on leased land, and no one knows what will happen when the lease is up or what rights buyers of cooperative shares in the building will have. The developers of the cooperative do not have an option to buy or to renew the lease when it expires 72 years from now.

When the lease is up, "we get the property back" including the building, said Stanley Westreich, a partner in Arland Towers Co., owner of the leased land.

Tenants opposing the conversion of the 1,655-unit, 27-year-old high-rise complex are doing so largely on more familiar grounds: the unwanted dislocation of tenants and the elderly who cannot afford to buy, the sudden need to find the money to buy for those who choose to, the loss of moderate-income rental housing, and suspicions over improvements promised by the developer.

But River Place adds a new and puzzling element to the typical list of conversion problems, one which Arlington County authorities say they have not had to deal with before: will buyers or, more likely, future buyers, have the land pulled out from under them when the lease expires? If so, what is the responsibility of the county or state governments, to only warn potential buyers of the new wrinkle in this plan or to do something about potential problems?

"Nobody knows exactly what the effect of this land lease will be. . . . Nobody knows what will happen to people's investment," said Nancy Johnson, Arlington County tenant-landlord coordinator. "There is some concern that as the lease date gets closer, it will be more difficult to mortgage the property."

"People who buy part of the cooperative won't be buying the land, only a share of the building. There is no other conversion like this in Arlington," she said. "Our initial concern is that tenants or other buyers might not understand this."

That fear has been alleviated by the developer's highlighting the land lease in its public offering statement, Johnson said, adding that "anybody who was buying carefully would see that provision ."

Still, it will be difficult for buyers to determine just what it might mean for them and their future investment, since even experienced housing officials in the county are unclear about it, and the developers are shrugging their shoulders about it.

Developer Gary Nordheimer, who with his brother Scott formed Monument Associates, is philosophical.

"First, none of us are going to be here in 72 years," Nordheimer reasons. He says Monument Associates tried to buy the land from its owner, but Arland Towers wouldn't sell.

Nordheimer said there is no way to tell what will happen eventually, because there has been no past experience from which to judge. He said similar land-lease arrangements have been tried in California and some of these leases are coming close to expiration now.

In these cases, Nordheimer said, the courts generally have told the buyers and the owner of the land to "work out some kind of business arrangement." These situations may give a clue as to where such ventures are headed, he speculated.

Arland Towers' Westreich agreed it's always possible that co-op buyers could arrange to buy the land when the lease is up, "but who knows what will happen in 70 years. . . . All they are buying is a right to live in the building until then."

Most people only live in a condominium here for three to five years, anyway, Nordheimer argued.

Tenant-landlord coordinator Johnson takes a longer-range view, however.

"Now they have 70 years, but say somebody 20 years old buys a place and wants to resell 60 years from now and move to a nursing home. Will a bank re-mortgage it with the lease due to expire in 10 years?" she asks.

The Arlington County Board is looking at the Arlington Towers conversion and has asked the developer to discontinue conversion work until it has looked into a number of issues raised by the tenants there.

On Jan. 5, the board passed a resolution expressing its "grave concerns over the manner in which the residents of Arlington Towers are being treated and questions the developers' compliance with current zoning and building codes."

It noted that the developers, who took the building over in September, have given tenants in one of the four buildings that comprise the complex to buy or move out by Feb. 28 and that other tenants have been given short-term notices of options to purchase. The board has scheduled a public hearing for Feb. 6.

At a working session this week to discuss the issue, tenants attacked the conversion on a variety of fronts.

Wallace Witkowski, an Arlington Towers tenant and vice chairman of the residents' association board, cited the difference between current rents and the cost of buying: His 11th floor one-bedroom apartment with a view of the Potomac now rents for $425, but if he bought the 716-square-foot unit it would cost $1,300 a month, he said.

With that kind of price, 90 percent of the current tenants would want to or would have to continue to rent somewhere, he said, adding that 22 percent of the residents are elderly or handicapped.

"There is a broader issue here . . . one of conversion of moderate-income housing to pseudo-luxury housing," Witkowski told the county board. The conversion "takes moderate-income housing and turns it into the singles-only world."

Nordheimer said that more than half the residents have incomes of more than $25,000 a year and could afford to buy, but Witkowski said the income figures were not accurate. When leases came up for renewal, tenants were told they had to have incomes of at least $20,000 to continue to rent even an efficiency, Witkowski said. This meant some lower-income people already moved out, while others falsified their income statements, he said.

The developer said there are only 26 elderly persons in the building and that they are being given special attention and concessions.

The residents also fear the developers could circumvent consumer protection provisions in the Virginia condominium conversion laws because River Place is a cooperative.

Nordheimer said the reason for making River Place a co-op rather than a condominium was that for a condo the developer needed the concurrence of the land owners and couldn't get it without agreeing to a large land-lease rent increase.

On complying with condo conversion laws, Nordheimer said, "We are doing as much as we can."